As performance advantages in semiconductor devices are identified, the speed of response in achieving these devices has been limited. Efforts have been undertaken to reduce the dimensions of the devices and to increase the carrier velocity so as to reduce the transit time of the carriers and thereby improve the speed of response. With such efforts, however, have come difficulty in manufacture and serious restrictions on the impressed voltages.
One particularly high-speed device known as a metal base transistor was reported in Proceedings of the IRE, Vol. 50, p. 1527, 1962. The device was composed of two Schottky barrier diodes back-to-back on a metal base.
An improved metal base type structure appeared in the art, as is described in copending patent application S.N. 118,171, filed on Feb. 4, 1980, the disclosure of which is incorporated herein by reference, wherein a two-stage emitter is provided in which, in the first stage, there is a high density of carriers and in the second stage, which is adjacent to a high conductivity base, there is a low barrier. The high conductivity base thickness is of the order of the mean free path of an electron.
A further development in this type of device can be found in U.S. Pat. application S.N. No. 371,849, filed on Apr. 26, 1982 to Solomon, disclosure of which is incorporated herein by reference, wherein the structure employs a field induced layer between two semiconductor regions of the same conductivity type, but of a different conductivity level to serve as a base region. The layer is produced by the field of the operating electrical bias that is applied to the device. The layer thickness is of the order of the mean free path of an electron so that ballistic-type transport is achieved.
A most recent development in this type of device is reported in IEEE Electron Device Letters Vol. EDL-6, No. 4, Apr. 1985, p. 178 wherein there is described a structure in which a base conductivity is induced by the collector field.
The structures in this type of device heretofore in the art exhibit less than optimum performance due to low current or voltage gain and low electron velocity during transit from the emitter to collector.